Log periodic dipole array with dual band directors



SPt 9, 1969 P. E. MAYES ETAL 3,466,655

LOG PERIODIC DIPOLE ARRAY WITH DUAL BOND DIRECTORS Filed Jan. 27. 19Go'United States Patent O 3,466,655 LOG PERIODIC DIPOLE ARRAY WITH DUALBAND DIRECTORS Paul E. Mayes, Champaign, and Ronald D. Grant, Ur-

bana, Ill., assignors, by mesne assignments, to JFD ElectronicsCorporation, Brooklyn, N.Y., a corporation of Delaware Filed Jan. 27,1966, Ser. No. 523,447 Int. Cl. H01q 11/10 U.S. Cl. 343-7925 6 ClaimsABSTRACT F THE DISCLOSURE An antenna array operating in both the VHF andUHF bands comprised of a dipole section arranged in logperiodic fashionand a director positioned in front of the forward end of the dipolesection. The dipoles each have a tip-to-tip electrical length ofone-half wavelength in the lower operating frequency range and furtherresonate at odd multiples of a half-wavelength. The directors each havean electrical length which is slightly less than threehalves wavelengthswhere the wavelengths are selected near the high end of the operatingfrequency range. The directors will further resonate at those oddmultiples of a half-wave length which are greater than three-halveswavelengths.

The instant invention relates to antennas and more particularly to noveldirector means for providing high-gain in antennas of broad-bandresponsive characteristics.

Antennas having broad-band response characteristics are quite desirablefor a variety of applications. For eX- ample, it is desirable to providea single antenna array for use in receiving (or transmitting) both thelow and high frequency television bands within the VHF frequency range.The television channels 2-6 lie in the band from 54-88 megacycles whilechannels 7-13 lie in the band VHF channel frequencies from 174-216megacycles. Broad-band antenna structures which have successfullyprovided reception (or transmission) for both the low and high bands ofthe VHF channel frequencies may assume a variety of designs. One designwhich has proven successful is that of the log-periodic antenna arraywhich is designed to resonate at a plurality of odd integral halfwavelength modes so as to provide reception (or transmission) for boththe low and high band VHF channel frequencies.

In many instances, such broad-band antennas lack adequate gaincharacteristics and one conventional approach which is employed toenhance the gain of the antenna is to provide parasitic elements in thearray.

Conducting rod or tubular elements incorporated in the antenna array andwhich are not connected to the feed line of the antenna array arecommonly used to increase the directivity and gain of the antenna. Suchelements are commonly referred to as parasitic elements so as todistinguish them from the elements which are electrically connected tothe antenna feed line. The latter elements are commonly referred to asdriven elements.

Parasitic elements which are placed -in front of the driven elements ofthe antenna array are commonly trimmed in length so as to resonate at aslightly higher frequency (approximately 5%) than the upper limit of thePatented Sept. 9, 1969 ICC frequency band of the antenna array. Thistrimming of the parasitic elements causes the phase of the current inthe parasitic element to be such that the field produced by theparasitic element adds to that produced by the driven element in thedirection proceeding from the driven element toward the parasite. Suchparasitic elements are commonly referred to as directors A plurality ofdirectors can be added to an antenna array to produce further increasesin directivity and gain, although the hierement in gain diminishes asthe number of directors increases beyond a predetermined number.

It has been conventional practice to employ directors having electricallengths which are slightly less than onehalf wavelength of the operatingfrequency. The phase of the induced current in the director element iscontrolled by the deviation of the element length from the resonantlength which, in the case of a director element, can be adjusted tocontribute to the electromagnetic field in the forward direction anddetract from the electromagnetic field in the backward direction andthereby enhance directivity of the antenna array.

At operating frequencies near the high end of the VHF rage and at UHFfrequencies and above, elements which are longer than one-halfwavelength are of a practical an d usable length. Such longer directorelements will contribute to the directivity and gain to an increaseddegree. Proper phasing of these elements can be achieved by operatingthe director elements at frequencies slightly below the second or higherorder resonant modes for the director.

It is therefore a primary object of the instant invention to employparasitic elements which are considerably longer in electrical lengththan one-half wavelength in their tip-to-tip developed length in orderto achieve signcant increases in directivity and gain for antenna array.

The instant invention is comprised of an antenna useable over either abroad or narrow band having an array of driven elements which aredesigned to provide good reception (or transmission) over apredetermined range of operating frequencies. The antenna array isfurther provided with at least one parasitic element having anelectrical length which is slightly less than an odd-integer multiple ofone-half the operating wavelength in total developed tip-to-tip length.Alternately stated, the parasitic elements employed in the instantinvention are most effective in increasing directivity and gain for theantenna array when they are operated at a frequency slightly below ahigher order of resonance of the dipole. The resonant length is selectedso that the second or higher order resonant mode for the parasiticelement lie near the high end of the frequency band to be covered andpreferably slightly higher than the highest frequency of operation ofthe antenna. The parasitic dipole may be comprised of a simpleconducting rod or tube element whose tip-to-tip length is appropriatelyselected so as to be resonant at a second or higher order resonant modewhich is preferably slightly higher than the highest frequency ofoperation of the antenna array. Since resonances above the lowest orderresonating mode become the operating resonances of the parasiticelements, the dipole halves are preferably inclined with respect to oneanother forming a V-dipole configuration in order to optimize theforward gain and hence the directivity of the antenna. Although thedirector elements are designed to operate near the second or higherorder resonant modes the lengths of these parasitic elements, even whenused in the upper part of the VHF frequency band are still of apractical length so as to provide an antenna which avoids the possiblemechanical problems in the design and support thereof. In cases where itis desired to shift the operating frequencies of the director elementsemployed, reactive elements may be added at the ends of the directordipole arms or along the length thereof so to provide director elementswhich have shifting electrical lengths for different operatingfrequencies.

It is therefore a primary object of the instant invention to provide anantenna having enhanced gain and directivity characteristics.

Still another object of the instant invention is to provide an antennacomprising director elements having electrical lengths greater thanone-half wavelength within the operating frequencies of the antenna toachieve increased directivity and gain for the antenna array.

Another object of the instant invention is to provide a novel antennaarray having enhanced gain and directivity characteristics and beingcomprised of at least one director element having an electrical lengthwhich is selected to operate the director element at slightly below thesecond or higher order resonant mode of the element.

Another object of the instant invention is to provide a novel antennaarray having enhanced gain and directivity characteristics and beingcomprised of at least one director element having an electrical lengthwhich is selected to operate the director element at slightly below thesecond or higher order resonant mode of the element and wherein thedirector element is further comprised of reactive elements for shiftingthe resonant frequency of the director element.

Still another object of the instant invention is to provide a novelantenna array having enhanced gain and directivity characteristics andbeing comprised of at least one director element having an electricallength which is selected to operate the director element at slightlybelow the second or higher order resonant mode of the element whereinthe director element arms are inclined with respect to one another inorder to optimize forward gain for the antenna array.

These and other objects of the instant invention will become apparentwhen reading the accompanying description and drawings in which:

FIGURE 1 is a schematic diagram showing an antenna of log-periodicdesign employing the director elements of the instant invention.

FIGURE 2 is a schematic diagram showing an alternative embodiment forthe antenna array of FIGURE 1.

FIGURE 3 is a schematic diagram showing a conventional Yagi antennashown for the purpose of describing the advantages of the instantinvention.

Referring now to the drawings, FIGURE 1 shows an antenna which iscomprised of a transposed .feeder line 11 having electrically coupledthereto arms 12a-14a and 12b-14b forming the dipoles 12-14,respectively. The arms of dipoles 12-14 are longest toward the left-handend of the array and decrease in length gradually toward the right-handend thereof. The take-off point 15 for Athe power, generally designatedby the symbol is electrically coupled to the transposed feeder harness11 and may be a power source (transmitter) or sink (receiver), dependingupon the particular application. In the preferred construction thespacing between adjacent driven dipoles measured along the longitudinalaxis of the antenna represented by the phantom line 16, decreases in agradual manner from the left-hand toward the right-hand end of theantenna array. The length of the antenna arms 12a-14a and 12b-14h andthe spacing between adjacent dipoles may typically be determined in themanner set forth in U.S. Patent No. 3,108,280 entitled Log-PeriodicBackward Wave Antenna Array, issued to Mayes et al. and assigned to theUniversity of Illinois Foundation.

Such log-periodic design yields an antenna array which provides constantgain characteristics over a wide range of frequencies. It is preferablethat the antenna be symmetrical about the phantom line 16 passingthrough the midpoints of the dipoles 12-14 and directors 17-19, to bemore fully described, The dipoles 1244 are preferably bent or inclinedin a V-coniiguration to enhance directivity of the antenna array whenthe dipoles are resonant in a higher order mode (3k/2, SM2, etc.).

When acting as a transmitting antenna, the antenna is fed at its narrowend from a conventional source of energy, depicted in FIGURE 1, by wayof illustration only, as alternator 20 which feeds the antenna by meansof the balanced feeder line 11 consisting of conductors 11a and 11b. Itcan be seen that the conductors 11a and 11b are crossed over one anotherbetween connections to consecutive or adjacent elements of the antennato provide a transposed feeder arrangement which greatly enhances theantenna front-to-back gain ratio as is well known in the prior art,thereby providing unidirectional response.

Although this type of driven section for the antenna array providesexcellent performance, the instant invention is in no way limited withrespect to the array of driven elements employed and may be employed toequal advantage with a driven section of another design. The explicitexample shown herein is merely presented for purposes of clarificationof the manner in which the parasitic elements of the present inventionmay be employed.

The parasitic elements 17, 18 and 19 are comprised of arms 17a-19a and17h-19h, respectively. Each of these arms may be separate conductingrods or tubes which are electrically connected at the inner ends andmechanically mounted along the antenna center line 16 in a V-congurationto provide optimum forward gain for the antenna array. If desired, thedirector elements 17-19 may be single straight elements which are bentat the longitudinal axis 16 for the antenna array 10 so as to providethe V- configuration. As can clearly be seen, none of these directorelements are electrically connected to the feeder line 11 and hence formthe parasitic section for the antenna array.

Although three such V-dipole directors 17-19 are shown in FIGURE 1, anynumber may be employed to advantage. The important criteria foroperation of the parasitic directors in the higher order modes is toadjust the second (or other higher resonant frequency modes of theelements to fall near the high end of the frequency band to be coveredand preferably slightly higher than the highest frequency of operationof the antenna. This may be done with either a simple conducting rod ortubular element by appropriate selection of the developed tipto-tiplength of the director element.

The principal advantages of the instant invention over conventionalmethods lie in the increased directivity and gain which the antennaarray yields through the use of the director elements. To illustrate bymeans of one application, consider the design of an antenna to cover theband of frequencies between 800-900 megacycles which is used for thetranslator stations in UHF television broadcasting. The driven sectionof the antenna array comprised of the dipoles 12-14 may be designed inaccordance with the formula set forth in the above mentioned U.S. Patent3,108,280. Let it be assumed that it is desired to operate the drivenelements to resonate at the 3M 2 mode. This means that the electricallength of the dipoles from tipto-tip will be approximately 21 inches.This length is the developed length of the dipole which is the lengthwhich the dipole would be if it were a straight dipole, i.e., before itis arranged in a V-conguration. At a frequency of 800 megacycles )t isequal to approximately 14.6 inches and hence 3M 2 is equal toapproximately 21.9 inches. At a frequency of 890 megacycles x is equalto 13.2 inches and hence 31/2 is equal to approximately 19.8 inches.Hence the electrical lengths of the driven dipoles 12-14 willapproximately lie between 19.8 and 21.9 inches. The

spacings between the driven elements 12-14 may be c stablished inaccordance with the equations set forth 1n the above mentioned U.S.patent, or in any other well known fashion.

In order to select the lengths of the parasitic elements 17-19, thesecond or higher order resonant mode of the driven elements should ibenear the upper end of the operating frequency range for the antenna andpreferably slightly higher than the highest operating frequency for thearray. The tip-to-tip developed length for the director element 17, forexample, would be approximately 19.5 inches. The remaining dipoledirectors 18 and 19 would also be o'f nearly the same tip-to-tipdeveloped length. Elements of these lengths pose no mechanical problemsin the construction of such an antenna while the beam width of a typicaldipole of this size is considerably less than that of a dipole operatingat the M2 resonant mode, thus greatly enhancing the directivity and gainfor the antenna array.

If it is desired to operate the director elements at higher resonantmodes, for example, at Slt/2 resonant mode, the tip-to-tip length forthe director element would be approximately 32.5 inches, which lengthlikewise poses no mechanical problems for the construction of suchantennas and which yields an even higher gain per element for theantenna array.

The parasitic elements 17-19 may be constructed of any good electricalconductors such as aluminum, copper, silver, gold, brass, just to name afew. The spacing between director elements when multiple 4directors areused maybe uniform or may be variable within limits. Generally, thespacing between the first director element 17 and. the adjacent drivenelement 14 will be different from the spacing between the rst directorelement 17 and the subsequent director elements.

If it is desired to shift operating frequencies for the directorelements this may be done in the manner shown in FIGURE 2. In theembodiment 10' o'f FIGURE 2, it can be seen that this arrangement issubstantially identical to the antenna array 10 of FIGURE 1 except that,the director element 17 has been provided with reactive members 21 and22 in the arms 17a and 17b, respectively, and director element 18 hasbeen provided with a reactive element 23 located at the apex of the arms18a and 18b and is electrically coupled to these arms. In thearrangement of director element 17 or of director element 18 thereactive elements 21-22 and 23, respectively, alter the resonantfrequency of the directors. Thev reactive component 23 likewise operatesin a similar manner. The reactive components may either be capacitive,inductive, or a combination thereof, depending only upon the objectivesof the user. A detailed description of the specific manner in which thereactive components are selected as to their values and are locatedalong the director element arms is set forth in detail in copendingapplication Ser. No. 414,975, entitled Multiple Mode OperationalAntennas Employing Reactive Elements, filed; Dec. 1, 1964 by Grant etal. and assigned tothe assignee of the instant invention.

To illustrate the superiority of the antenna array described herein whencompared with conventional methods, consider the conventional designshown in FIGURE 3. The antenna array 30 of FIGURE 3 is a Yagi antennaemployed for use in the translator frequency band of 800l890 megacycles.The Yagi antenna configuration 30 is provided with a single drivenelement 31 having dipole. arms 31a and 31b. A single parasitic element32 serves as a reflector and the elements 33-40 serve as directors. Thedriven element 31 is operated near the first resonant mode, i.e., the M2resonant mode in order to provide a resistive input impedance to matchthe transmission linc 41. The reflector element 32 is of an electricallength which is 4slightly longer than the driven element and thedirectors are slightly shorter than the driven element 31 to provide theproper phasing for currents induced within the elements. Hence, allelements of the Yagi antenna are approximately one-half wavelength intip-to-tip dimension. At the UHF translator frequencies this length isapproximately seven inches. In order to achieve the high-gain, which isvery desirable at these frequencies, it is necessary to use a very largenumber of directors. This leads to an antenna which may be six or sevenfeet long and only seven inches wide. Furthermore, after adding a fewdirector elements the increment in gain which is achieved by adding eachadditional director becomes quite small. It can therefore clearly beseen that a comparison of the conventional design antenna 30, shown inFIGURE 3, with an antenna array employing the director elements of theinstant invention, that gain and directivity characteristics are quitesuperior through the use of the design of the instant invention, whilethe antenna array is much simpler and does not provide the mechanicalproblems which are confronted through the design of the Yagi antenna ofFIGURE 3.

It can therefore be seen from the foregoing that the instant inventionprovides an antenna array employing director elements which aresubstantially longer than the director elements of conventional practiceso as to greatly enhance the directivity and gain characteristics forthe resulting antenna array.

Although there has been described a preferred embodiment of this novelinvention, many variations and moditications will now be apparent tothose skilled in the art. Therefore, this invention is to be limited,not by the specic disclosure herein, but only by the appended claims.

What is claimed is:

1. An antenna array comprised of:

a driven section having a plurality of dipoles;

a transposed feeder line for electrically coupling all of said dipoles;

said dipoles being spaced relative to one another and each havingelectrical lengths of odd multiples of a half-wavelength selected tooperate said array in logperiodic fashion over a substantially broadfrequency range;

the dipoles of greater electrical length being located toward the rearof said driven section and the dipoles of shorter electrical lengthbeing located toward the front of said driven section;

at least one director element being positioned in front of said drivensection;

said director element having Va tiptotip electrical length l, where l isslightly less than (2n-|-1))\/2; where nl is any non-zero integer; andWhere )t is the wavelength for a frequency at the high end of theoperating frequency range, the length of said director element beingless than the length of the shortest dipole.

2. The array of claim 1 wherein said director element is comprised offirst and second sections symmetrically arranged about said longitudinalaxis;

said sections being aligned to form a V-coniiguration with said sectionsbeing directed substantially toward the front of said array.

3. The array of claim 1 wherein said director element is comprised offirst and second sections and is further provided with at least onereactive element electrically coupling said sections;

said reactive element being positioned intermediate the tips of saiddirector element;

said reactive element being provided for shifting the resonantfrequencies for said director element.

4. The array of claim 1 wherein a plurality of director elements areprovided; each of said director elements being comprised of first andsecond sections symmetrically arranged about said longitudinal axis;

said sections being aligned to form a V-coniiguration with said sectionsbeing directed substantially toward the front of said array.

5. The array of claim 4 wherein at least one of said director elementsis provided With at least one reactive element electrically couplingsaid sections;

said reactive element being positioned intermediate the tips of saiddirector element;

said reactive element being provided for shifting the resonantfrequencies for said director element.

6. The array of claim 4 wherein at least one director element isprovided with first and second reactive elements each being positionedalong an associated element section intermediate the longitudinal axisof said arr-ay and the tip of the associated section for shifting theresonant frequencies of said one director element.

References Cited UNITED STATES PATENTS 2/ 1968 Simons 343-815 6/ 1964Greenberg 343-7925 10/ 1949 Carter 343-915 X 12/ 1949 Busignies 343-8546/ 1953 Middlemark 343-819 X 4/ 1957 Marshall 343-815 4/ 1961 Anderson343-815 U.S. Cl. X.R.

